Re: plaster ceiling on 100 yr old house
It's possible that the hairline cracks have been the for a long long time, you just didn't see them. The leak caused them to be discolored with dirt or mineral stain and more visible. Plaster usually holds up well to some leakage a lot better than drywall.
If the plaster is tight to the ceiling, if it were me, I would do nothing until every thing is completely dried out. Then try painting an area first with oil based Kilz to seal and block stain then a coat of paint. The cracks may just disappear.
If the cracks are still viable you could have painters cloth installed over the plaster and paint rather than pulling it down or have a plasterer skim coat the ceiling.
Be sure you live your life, because you are a long time dead.-Scottish Proverb